GPR and EM31 investigations on an active CO2 gas vent

The goal of CO2 geological sequestration is permanent isolation, however there is always the potential that CO2 may eventually leak to surface. It is thus critical to understand what effects leaking gas may have in the shallow environment, and the best way is to study natural analogues where geologically produced CO2 is already leaking to surface. Geophysical and geochemical investigations were undertaken on a CO2 gas vent in order to better understand its morphology, alteration and gas flow regime. A total of 61 parallel GPR profiles, using 250 MHz antennae, were performed at 1m intervals across the vent to obtain a 3D image. Frequency Domain Electromagnetic (FDEM) data has been obtained for 0-3 and 0-6 depth intervals over the same area as the GPR, while some TDR measurements were also performed in order to better understand the shallow GPR data. GPR and EM31 results both demonstrate a highly anomalous sub-circular area which corresponds with the non-vegetated core of the gas vent. These two surveys also show a much larger anomalous halo around the core which corresponds well with soil-gas geochemical results, particularly with elevated CO2 concentrations. It is suggested that water-gas-rock reactions have modified the mineralogy of these anomalous zones, thereby resulting in a change in their electrical response.